During normal insertion of an IV (intravenous) catheter, positive indication of entry into a vein is shown by blood entering the normally transparent needle hub through the needle. So long as the needle and catheter are in the vein the chamber will continue to fill with blood. If the needle and catheter are pushed in too far, thereby puncturing the distal side of the vein wall and not remaining in the vein lumen, the blood will stop filling the flashback chamber (bore) of the needle hub.
In some medical situations an anesthesiologist will wish to use a 16 gauge or larger catheter. In this case, the large needle size allows blood to fill the flashback chamber so quickly that it is difficult to determine whether the needle is in the vein lumen or has passed through the distal vein wall. Normal practice is to add a syringe to the flashback chamber to provide for extra volume. This is costly and requires the procurement and storing of extra parts. In some cases the syringe plunger is removed making the larger flashback chamber essentially an open reservoir to the air. This can increase the chance of infection.